A central disability associated with post-stroke hemiparesis is that muscle excitation is impaired, and thus muscles fail to produce properly graded and timed force. With impaired muscle coordination, walking subtasks required to achieve normal speed, such as trunk forward progression (acceleration), swing initiation (kinetic energy of the leg at the end of stance) and muscle power generation, may not be satisfactorily performed. The relationship between impaired muscle force production and walking speed in persons with hemiparesis is unknown, yet this is extremely important for the design of effective gait rehabilitation strategies in hemiparetic populations. In the proposed study, experimental data from individuals with post-stroke hemiparesis across a range of functional walking levels and speed-matched healthy elderly individuals will be combined with a theoretical modeling framework based on dynamic simulations to quantify muscle force contributions to walking. The PIs propose to use this scientific model-based framework to understand and quantify how abnormal muscle force production reduces walking speed in post-stroke gait disorders. Specific aims are to (1) determine how deficits in trunk forward progression, swing initiation and muscle power generation are caused by abnormalities in muscle force production during pre-swing in the paretic leg, and how abnormalities during concomitant early stance in the non-paretic leg compensate; and (2) determine how muscle coordination must be improved for subjects with hemiparesis to increase their functional walking status. The long-term objectives of the proposed work are to show that measures of abnormal muscle coordination in post-stroke walking will predict the outcome of therapy, assist in defining the specific muscle coordination changes associated with various therapeutic interventions, and correlate with structural and functional studies of the nervous system such that the underlying mechanisms can be better understood.